1) Field of the Invention
The present invention relates to a dispersion-slope compensator which compensates for dispersion slope, i.e., wavelength dependence of chromatic dispersion.
2) Description of the Related Art
The optical communication networks can be a core to form a base of a communication network, and it is desired that the services of the optical communication networks become available in wider areas and further sophisticated. In particular, development of the WDM (Wavelength Division Multiplex) technique which constitutes a core technology for constructing optical communication systems is rapidly proceeding. The WDM is a technique in which a plurality of signals are concurrently transmitted through a single optical fiber by multiplexing light having different wavelengths.
Since the transmission velocity of light through an optical fiber is different depending on the wavelength of the light, the deformation (broadening) of light pulses caused by chromatic dispersion increases with the transmission distance. The chromatic dispersion can be defined as a difference between the times taken for monochromatic light of two wavelengths 1 nm apart to propagate for 1 km, and is expressed in ps/nm/km. For example, in the case where light with the wavelength around 1.55 micrometers propagates through a single mode fiber (SMF), the chromatic dispersion is 15 to 16 ps/nm/km.
When broadening of a light pulse is caused by chromatic dispersion in a WDM system, which realizes a large-capacity, long-distance optical transmission, the reception level seriously deteriorates, and the deterioration of the reception level adversely affects the system. Therefore, it is necessary to compensate for dispersion so as to substantially eliminate the chromatic dispersion occurring in the optical fiber by adding a chromatic dispersion which is equal in the magnitude and opposite in the polarity to the chromatic dispersion to be eliminated.
Currently, the most frequently used means of compensating for dispersion is the dispersion compensating fiber (DCF), which is designed to cause an opposite structural dispersion to the material dispersion of the fiber material used for forming the SMF by realizing a specific distribution of the refraction index. Since the SMF has positive dispersion, the DCF is normally designed to have negative dispersion.
Conventionally, in the case where dispersion in a WDM system is compensated for by using a DCF, a DCF which compensates for chromatic dispersion in a central channel in the WDM system is connected to an SMF for compensating for chromatic dispersion in optical signals around the central channel, and the residual dispersion (portions of the chromatic dispersion which are not compensated for by the DCF are compensated for by using dispersion compensators provided in a receiver station.
For example, in the case where forty wavelengths are multiplexed in the WDM system, a DCF which compensates for chromatic dispersion in optical signals in the twentieth channel (as the central channel) is connected, and dispersion compensators are provided in a receiver station for compensating residual dispersion.
A typical one of the dispersion compensators is the chirped fiber Bragg grating (CFBG). The CFBG is an optical fiber in which a diffraction grating is realized in the core of the optical fiber by forming periodic variations of the refraction index in the core. Wavelength components of light inputted into the CFBG are differently delayed when the light propagates through the CFBG, so that both of the positive and negative chromatic dispersions can be compensated for. The CFBG is used in combination with an optical circulator or the like.
Further, in a conventional technique for compensating for dispersion which is disclosed, for example, in Japanese Unexamined Patent Publication No. 2000-151513 (Paragraph Nos. <0052> to <0059> and FIG. 9) corresponding to U.S. Pat. No. 6,289,151 (from column 9, line 10, to column 10, line 2, FIGS. 9A and 9B), desired phase responses are applied to inputted optical pulses by using a coupled-type ring resonator as an all-pass optical filter so as to compensate for dispersion.
However, in the aforementioned conventional system in which dispersion is compensated for by using the DCF, the dispersion compensators for compensating for the residual dispersion are required to be provided for the respective channels. Therefore, a great number of dispersion compensators of a great number of different types are necessary, and thus it is impossible to economically construct a WDM system with such a great number of dispersion compensators.
The causes of the residual dispersion are considered below.
In order to consider the characteristics of the chromatic dispersion, the wavelength dependence of the chromatic dispersion, which is called dispersion slope and expressed in ps/km/nm2, is important as well as the amount of chromatic dispersion.
In order to compensate for dispersion, the dispersion slope over the entire wavelength range of signal light is also required to be compensated for as well as the amount of dispersion. However, since the DCF and the SMF have different dispersion slopes, the portions of chromatic dispersion which cannot be compensated for remain in near-end regions of a wavelength band used in the WDM transmission (for example, the so-called C band, which extends from about 1,525 nm to 1,565 nm). Therefore, the residual dispersion accumulatively increases with the transmission distance, and it is necessary to compensate for the residual dispersion for each channel on the receiver side.
FIG. 24 is a graph illustrating a residual dispersion. In FIG. 24, the abscissa corresponds to the wavelength (nm), and the ordinate corresponds to the chromatic dispersion (ps/nm). FIG. 24 shows the residual dispersion in a WDM signal in which forty waves in channels ch1 to ch40 are multiplexed.
When a WDM signal in which the forty waves in channels ch1 to ch40 are multiplexed in the C band with channel spacings of 100 GHz is transmitted through an SMF for 10 km, and undergoes dispersion compensation realized by a DCF which is designed for the central channel ch20 to have a dispersion slope of 0.2 ps/km/nm2, the residual dispersion is approximately +20 (ps/nm) in the channel ch1, and approximately −30 (ps/nm) in the channel ch40 although the residual dispersion in the channel ch20 is eliminated.
FIG. 25 is a diagram illustrating a conventional WDM system. In FIG. 25, only the configuration for one-way transmission is indicated. The WDM system 100 illustrated in FIG. 25 multiplexes at most forty waves, and comprises stations 110 and 120 and repeater amplifiers 130-1 to 130-6. In addition, the optical fibers used in the transmission line are SMFs, and dispersion compensation filters (DCFs) f1 to f6 are arranged in the repeaters. Each of the DCFs f1 to f6 is coiled, contained in a small package, and arranged in one of the repeaters as an optical component.
The station 110 comprises optical transmitters 111-1 to 111-40, a wavelength multiplexer 112, and a WDM amplifier 113, and the station 120 comprises optical receivers 121-1 to 121-40, a wavelength demultiplexer 122, and dispersion compensation modules (DCMs) 123-1 to 123-40.
In the station 110, the optical transmitters 111-1 to 111-40 respectively output optical signals in the channels ch1 to ch40, the wavelength multiplexer 112 multiplexes the optical signals in the channels ch1 to ch40 and generates a WDM signal, and the WDM amplifier 113 amplifies the WDM signal and outputs the WDM signal onto the transmission line.
The repeater amplifiers 130-1 to 130-6 amplify and relay the WDM signal transmitted through the SMFs, and the DCFs f1 to f6 cause dispersion which compensates for the dispersion occurring in the channel ch20. When the optical signals in the channels ch1 to ch40 pass through each of the DCFs f1 to f6, the chromatic dispersion occurring in the preceding SMF is eliminated.
In the station 120, the wavelength demultiplexer 122 demultiplexes the WDM signal into the forty optical signals in the channels ch1 to ch40, and the DCMs 123-1 to 123-40 arranged in correspondence with the channels ch1 to ch40 compensate for the residual dispersion in the channels ch1 to ch40, respectively. The optical receivers 121-1 to 121-40 performs reception processing of the optical signals after the residual dispersion is compensated for.
FIG. 26 is a dispersion map for the WDM system 100 illustrated in FIG. 25. The dispersion map M1 illustrated in FIG. 26 indicates that the positive dispersion occurring in the channel ch20 through the SMF is compensated for by the negative dispersion caused by the DCF in each repeater section. Therefore, the dispersion in the channel ch20 is eliminated in each repeater section, and the dispersions in the channels adjacent to the channel ch20 are also within a certain dispersion tolerance.
However, since it is impossible to completely compensate for the dispersions occurring in the outermost channels ch1 and ch40 located at the ends of the wavelength range by the optical transmission line which is designed for dispersion management of the channel ch20, the residual dispersions in the outermost channels ch1 and ch40 greatly go out of the dispersion tolerance. Therefore, it is necessary to provide in the station 120 a DCF for each channel in which the residual dispersion goes out of the dispersion tolerance. Thus, in the system illustrated in FIG. 25, the DCMs 123-1 to 123-40 are provided for all the channels ch1 to ch40.
When the transmission rate increases, for example, from 10 Gb/s to 40 Gb/s, the dispersion tolerance decreases, i.e., the aperture in the eye pattern becomes smaller. Since the waveshapes of the optical signals received by the optical receivers 121-1 to 121-40 deteriorate, and the possibility of erroneously determining the signal values (i.e., the symbol error rate) increases, high-precision dispersion compensation is required when the transmission rate increases.
As explained above, since the DCFs and the dispersion compensators for the respective channels are arranged for dispersion compensation, the equipment size increases, and construction of an economical network is difficult. In addition, the greatness in the number and the number of types of dispersion compensators imposes heavy loads on the designer of the dispersion management system.
On the other hand, since the aforementioned CFBG is a dispersion compensator for a single channel, it is impossible to compensate for the residual dispersions in all the channels by one operation. Further, since the range of controllable amounts of dispersion slope is small according to the aforementioned technique disclosed in Japanese Unexamined Patent Publication No. 2000-151513 by C. K. Madsen et al., it is impossible to arbitrarily set the amount of compensation for the dispersion slope. Therefore, the technique by C. K. Madsen et al. cannot be applied to the DWDM (Dense-WDM) systems performing transmission with a high transmission rate such as 40 Gb/s or a further higher transmission rate on the order of a petabit per second. Thus, it is difficult to expect further development of this technique for construction of next-generation multimedia networks.